There are some written guidelines. The CDC "uses a blood lead reference value (BLRV) of 3.5 micrograms per deciliter to identify children with blood lead level

There are some written guidelines. The CDC "uses a blood lead reference value (BLRV) of 3.5 micrograms per deciliter to identify children with blood lead levels that are higher than most children’s levels." (https://www.cdc.gov/nceh/lead/prevention/blood-lead-levels.htm https://www.cdc.gov/nceh/lead/prevention/blood-lead-levels.h...). The difference between lead and sunlight as described is that there is likely some lower bound with sunlight as well, too little sunlight is unhealthy, which makes the advice "all sun exposure is bad" not only unhelpful but possibly unhealthy.